APPENDIX
C
261
emphatically
that under the conception of an antithetic alternation of generations all the
phenomena observed in the development of any higher animal, from a hydroid
polype to man himself, are capable of ready and simple explanation. On this
view of matters it is clear why in the Nemertean life-cycle the worm should
oust the Pilidium larva, why the starfish should substitute itself for
the Bipinnaria, why two distinct and separate nervous systems should
appear in a certain definite order in any development, why these should differ
so markedly in morphological features as do that of the trochophore and
that of the annelid, or as do the transient nervous apparatus of the skate
development and that of the sexual generation, the skate itself. It explains
why there may be two distinct nervous systems, and not three, four, or more.
The doctrine of direct development is impotent in face of these and similar
facts. But possibly there is a sort of explanation given by the recapitulation
theory, under which every animal recapitulates more or less completely and
perfectly its ancestry in the course of its individual development. There are
various difficulties in this theory—in fact, it bristles with them. Like many other
current beliefs, it was originally never founded in observation at all, and it
is still largely advocated in spite of all observation. Were there a basis of
truth in it, it ought to be easy for any embryological upholder of the doctrine
to give an outline of, say, the ancestral history of the vertebrata, or of the
mammalia. For the latter class, since the days of the late Professor W. K.
Parker, no one except Haeckel has ever attempted to explain the facts in the
light of this theory while for the former, the name of the theories supposed to
be founded in recapitulation is legion. Moreover, each excludes the
others.
But
suppose recapitulation were really the true explanation of the facts of
development, and not, as it actually is, the illusion of the human imagination:
under it there would be happening daily in every development an alternation of
generations, beside which that